Our project aims to elucidate mechanisms by which an oncogenic process inhibits the immune defensive mechanism of the host organism. In particualr, in the next period of the project, we will study the nature and mode of action of various types of immunosuppressive cells which appear during tumor development. Such cells have been observed in our laboratory as well as by others and will be studied in the model of oncornavirus-induced leukemias in mice. BIBLIOGRAPHIC REFERENCES: Cerny, J., M. Essex, M.A. Rich, and W.D. Hardy, Jr. Expression of virus associated antigens and immune cell functions during spontaneous regression of the Friend viral murine leukemia. Int. J. Cancer, 15:351 (1975). Cerny J. and E.B. Waner. Specific susceptibility of sensitized (memory) B cells to suppression and antigenic alteration by murine leukemia virus. J. Immunol., 114:571 (1975).